Method for curing rubberlike materials



April 1953 s. R. PRANCE 2,635,293

METHOD FOR CURING RUBBERLIKE MATERIALS Filed Feb. 15, 1950 l I o 0 l I oI l c h o *s FIJ 6; nw/z/vro/z 5mm AMA/vet l BY 1 g: H/J Arm/m YaPatented Apr. 21, 1953 METHOD FOR CURING RUBBERLIKE MATERIALS Stanley R.Prance, Dayton, Ohio, assignor to General Motors Corporation, Detroit,Mich., a

corporation of Delaware Application February 1 8 Claims.

This invention relates to a method for curing rubber and rubber-likematerials and is particularly concerned with a method for curing suchmaterials at high temperatures. I

It is, therefore, an object of the invention to provide a method forcuring rubber and rubberlike materials which are capable ofvulcanization under accelerated conditions of time, through the use ofhigh temperature curing procedures. In carrying out this object, it is afurther object of the invention to cure the rubber-like materials attemperatures of from 500 F. to 900? F., whereby the cure is effected ina matter of from 1.25 to four or five minutes.

It is a further object of the invention to'cure such materials in acontrolled atmosphere whereby the rubber or rubber-like material is notcharred or otherwise deleteriously effected by the elevatedtemperatures.

A still further object, in some cases, is to provide a method for curingrubber and rubberlike materials at elevated temperatures and, in shortperiods of time wherein steam is introduced into the curing chamber forproviding an atmosphere substantially non-oxidizing to the surface ofrubber or rubber-like materials.

A still further object of the invention is to provide a quench for therubber-like material after curing whereby the cured material reduced intemperature prior to exposure to the atmosphere for eliminatingoxidation of-the surface thereof. x

.A still further object, in some cases; is to provide a quench whichconditions the material for providing desired surface characteristics,for example, glycerine and water quench.

Further objects and advantages of the present invention will be apparentfrom thezfollowing description, wherein the drawing showsdiagrammatically a curing apparatus for extruded material.

Curing of rubber or rubber-like materials is a well known expedient inthe art. Cures are usually carried out at temperatures in .the order of300 F. to 350 F. either in open steam or in closed molds wherein thecure requiresfrom fifteen to thirty minutes for completion. The cure iseffected through normal vulcanization of the product as is well known inthe art wherein a vulcanizing agent such as sulphur with or withoutsmall additions of zinc oxide is added to the rubber or rubber-likecompound.

In this connection, any compoundcapable of vulcanization may be cured inthe process and apparatus to be explained in detail hereinafter.

5, 1950, Serial No. 144,238

2 Specifically natural rubber, butadiene-st'yrene copolymers,butadiene-acrylonitrile copolymers, polychloroprene and compatiblemixtures of any of the foregoing are illustrative of some of the manytypes ofrubber and rubber-like materials which may be cured. It is to beunderstood that the particular compound forms no portion 'of thisinvention and that the only limitation in this respect is that thecompound of the rubber or rubber-like material will be capable ofvulcanization.

The method as disclosed herein is best adapted to continuous processing,such as, continuous curing of the product of a; tuber or other type ofmachine,-although it will'be apparent that articles may be cured in thesame manner in the same curing oven or retort. i One type of apparatusisshown diagra'mmath cally at 20 and consists of an elongated tunnellikeoven 22 having anendless conveyor'24 pass?- ing therethrough. The oven22 is preferably insulated, not shown, to conserve heat'andxis ventedthrough a stack 26 which includes a draft control damper 28 therein. Theoven or chamber 22 includes an inlet 30 and an outlet 32 adjacentopposite ends thereof and through which the rubber-like material may bepassed. A tuber head shown at 34, produces a continuous formed length ofuncured rubber-like material 36 which passes directly through a solution38 of soap or other material to prevent subsequent sticking andthen'pa'sses through the entrance 3!] on the conveyor 24 into the oven22. After passing the entirelengthof the oven 22, therubber-likematerial passes through exit 32 into a tank 40 which includesa quenching liquid42 therein. Preferably a tunnel-like bafile or tunnel44 is provided to assure that the rubber-like material passes below the.surface of the liquid 42 without.ex posure to the atmosphere.

In order to heat the oven 22, a gas fired. unit 46 is provided which hasan outlet 48 connecting to the-chamber 22. The unit 46 burns gas and maybe adjusted for incomplete combustion thereof whereby atmospheres may beproduced having predetermined and desired quantities of carbon monoxide,carbon dioxide, oxygen, etc. Thus the unit 46 not only acts as a heaterfor the chamber. 22 butalso as a gas producer for supplying a desiredand controlled atmosphere to the chamber 22. The unit 46 is preferably aforced draft device wherein a blower (not shown) produces a continuousflow of heated gas through the connection 48. In some cases, it may ;bedesirable to use a regenerating process any desired quantities withatmospheric air for primary and secondary air for the unit 46.

A branch pipe 52 is also providedwhich hasa plurality of lead-oifs 54preferably evenly spaced throughout the length of the chamber 22.- Pipe52 carries steam which is exhausted; throughthe lead-ofis 54 into theoven 22.

The heater unit 46, in some cases, may nothave the capacity to bring thetemperature "of the'oven to the desired point in which event electricalbooster units, not shown, may be provided with- .in the-chamber 22.Similarly, if the unit 46 is-used merely as a gas producing unit tosupply -vcontrol-led atmosphere only to the oven, it is possible tocarry out the-entire heating of the oven H th-rough the use ofelectrical heaters or other suitable means.- In other words, it is to beunderstood that the oven 22 may be heated in any desiredmannerand thatthe controlledatmos- ,phere may be supplied thereto in any desiredmanner to-accomplish the-results to bestated hereinafter. n

The basis for the present curing method-resides in--thehigh-tem-peratures utilized during thecure. We have foundthattemperatures ranging-"from 500 F. to 900 .F; form asuitable rangein-whichrubber' 31'1dfl11bb8lf-1lke materials may be curedwithoutdeterioration thereof eitherby charringor other deleterioussurface effects: This is" accomplished through the use ofa controlledatmosphere within the oven whichmaybelsupplied, as noted, by-the-unit 46or may be supplied-from tanked gases, etc. We ,have'foundthat if theoxygen content within theoven is" maintainedwithin' predeterminedlimitsj that the'atmosphere is'substantially nonoir'idi'zin'g incharacter to rubber-like material. When the atmosphere contains oxygenover "8 there is-atendency, asthe high end of the tam-'- p'erature rangeis approached, for the rubber to cha r and otherwi'sedisintegrate; Ifthe oxygen iii theat'mosphere ismaintained at 8% or less, this tendencyissubstantially eliminated at the upperend of the temperature range. Theok'i geh cohtent may'beas high as 14% toward the' lower-end ofthetemperature ra'nga Stated differently, for a temperature range offrom 500F; to 900 'E'the oxygen content'must vary inversely from 14% to8%. In this'connection, 'if' only 'air'is used as the oven atmosphere,the rubber-like material, being cured,-will actually char and/or "burn;In place of anatmosphere produce-d by'a unit, such as unit 46, whichatmo'sphere is essentially incompletely burned, natural gas, nitrogen,carbon dioxide, carbon monoxide, mixtures thereof together or with air,maya-ube used providing the oxygen content 01" the complete mixture ism'aintainedwithin the'r'a'nge of 8%to 14% depen'ding'onthe'temperarturelIt is apparent-that any o'fthese-gases maybe used in pure 'form withequally good results" although the expense is muchgreater'.

Wehavefurther found that the introduction of steam has abeneficiareffect on the cure and "t'otliisend-we sometimesintroducesteam into the oven. This introduction is not entirely 4necessary but it supplements the atmosphere with a non-oxidizing gas andthereby aids in cutting the cost of the operation.

In general, the cure requires about 4 minutes at a temperature of 500 F.about 2.5 minutes at a temperature of 600 F., about 1.5 minutes at atemperatureaof 3002B, about li-25 minutes at the temperature .of 900with the .600 F. temperature being the preferred range.

I In all cases, it is desirable to cool the cured rubber prior toexposure of atmospheric air and thus mini-mize' su rface oxidation. Thisis most easily accomplished by merely passing therubber-part-directlyfrom the oven into a quenching tank' filled withwater or other liquid to be discussed-later. This quench cools theexternal surface .of the vulcanized part rapidly thereby preventingsurface oxidation but probably does noticompletely cool the internalportions of the part, whereby it may be possible that some furthercuring'action takes place after thematerial comes from the quench.InsteadIof-water, a glycerine mixture may be used in the quenchillg tankwhich produces desirableesurfaceecharacteristics on the quenched part.-Also other organic alcohols may be used-for specific results.Theapparatus, as diagrammatically shown-herain, is synchronized withthetuber head-M-so'that no stretching of thetubedpart :OCGLlI-Sf duringthe transfer thereof-from tank 38 to conveyor 24; etc. Also,the'catenary, noted the'quenching. tank, should be controlled sothatr-thepart is suflieiently cooled to preventanywstretching, Thissometimes requires the use of a longerwtank than shown-wherein thecooled material: may becarried onrollers at the bottom of the tank-:-

The method" shown herein is at-remendous improvement over conventionalcuring processes in that the time required ,forthe-cnreeisgreatlyreduced, thereby. permitting considerablyhigher production to be runthrough a given oven. Actual tests; such as, resiliency, aging-characteristicsa and other physical -properties; and the like of materialscured'by' theaccelerate'd process described hereinare equally asgoodia'sthose of materials cured :under standard curing conditions.

It is to be understood that-any ofwthe :conven' tional-rubber-or-*rubb'er like 'ec'o'mpound's; capable of-vulcanization; canbe used in this process'e- In this connection, however, anincrease inth'e amount of accelerator used is desirable? In general, accelerator inquantities of from 2 to 6 times the normal quantities is recommended.

A specific example in thecuring of a sealing strip wherein the strip hasa 'maximum thick nesscross section ofapproximately 4 o'f an inchis asfollows: The 'stock usednsu ator mula'ofi Per cent Butadiene-styrenecopolymer, 30.5% ,395 Natural rubber, 9% Lnnl; Zinc oxide 2 carbon black----422 Anti-oxidant (AgeriteWhite) ,-.e. a2 wax '(Micro cryst'alline) i3 Plasticizer-' (mineral'oil) 7L5 Cuma'nresin I 4,5 Accelerator(zimate') 1:1

This stockwas properly preparedand then- 99.1% of the stock had addedthereto .9 o'f 'a' percent-of sulphur just prior to tubing.

The stock above noted-was tubed mm a strip which was' carried throughsoap water-into the oven at a' rate of from to: feet pervminute. Theovenis about feet longand temperature therein wasma'intained at from 575F.;to-6251F. The atmosphere analysis during the cure was as follows: -i

Nitrogen, the remainder.

The rubber strip," after curing, was carried directlyinto a tank ofwater 15 feet in length) maintained at a temperatureirom 55.-to 7 5, F.Thus the time required for curing any givenwpart ofr the tubed strip wasin the order ofrrrom 2 to-3 minutes. V i v Similar satisfactory resultshave been obtained with a wide variety of cross sections ofmaterial. H

It is to be understood that the speed of travel must be correlated tocompensate for the temperatures involved-and maximumthickness of crosssections ofmaterial being cured in order to obtain satisfactory cures.Thus, it is advisable to make a test run on each new cross section ofmaterial in order to assure proper curing thereof. In all cases,however, when using the elevated temperatures under controlled atmospheric conditions, it is possible to eiTect the cure in a short periodof time ranging from 1% to 4 or 5 minutes.

As previously mentioned any vulcanizable ruboer-like compound can becured in the manner proposed herein, that is, compounds embodying theusual rubber-like materials, vulcanizing agents, accelerators, andanti-oxidant, etc. Specifically, the following formula gives rangeswhich yield satisfactory results:

Parts Rubber-like polymer or copolymer 100 Vulcanizing agent (sulphur,zinc oxide, fatty acid) 1 to 10 Pigment and/or filler Oto Anti-oxidant0.5 to 3 Accelerator 0.5 to 8 To this material may be added aplasticizer when necessary. This is particularly desirable when thematerial is to be tubed to soften the might be adopted.

What is claimed is as follows:

1. In the method of curing compounded rubber-like materials including avulcanizing agent, the steps comprising; shaping vulcanizable uncuredrubber-like material, supplying said material to an oven, heating thematerial at, a temperature of at least 500 F. and not more than 900 F.at a pressure slightly in excess of atmospheric pressure in an amosphereof incompletely burned natural gas containing from 14% to 8% oxygenvarying inversely as the "6 temperature andfor a time ranging from'about4 minutes to 1.25 minutes, and then immediately cooling the vulcanizedpart undernoni-oxidizing conditions before exposure to the atmosphere.

2. In the method of curing compoundedrub+ ber-like materials including avulcanizing agent, thesteps comprising; shaping vulcanizable uncuredrubber-like materiaL supplying said ma terialto an oven, heating thematerial at a'temperature in the neighborhood of. 600 .F., at a pressureslightly in excess of atmospheric pressure in an atmosphere which isnon-oxidizing to the material and wherein'the oxygen content is in orderof and does not exceed 14%, for about 2.5 minutes, and then immediatelycoolingthe vulcanized part undernon-oxidizing conditions before exposureto the atmosphere. 1

In the method of curing compounded rubher-like materials including avulcanizing agent, the steps comprising; shaping 'vulcanizablei'uncuredrubber-like material, supplying s aid ma terial'to an oven, heating thematerial at atemperature of at least 500 Ffiandnot in excess of 600 F.at a pressure slightlyin excess 'of'atmospheric pressure for atimeranging from 4 minutes to 2.5 minutes and in an atmosphere containingbetween 14% and 8% oxygen, and finally cooling the vulcanized partbefore exposure to the atmosphere.

4. In the method of curing compounded rubber-like materials including avulcanizing agent, the steps comprising; shapingvulcanizable uncuredrubber-like material, supplying said material to an oven, heating thematerial at a temperature of at least 500 F. and not more than 900 F. ata pressure slightly in excess of atmospheric pressure in an atmospherecontaining oxygen within the range of 14% and varying inversely as thetemperature and for a time ranging from about 4 minutes to 1.25 minutes,and then immediately quenching the vulcanized part in water.

5. In the method of curing compounded rubber-like materials including avulcanizing agent, the steps comprising; shaping vulcanizable uncuredrubber-like material, supplying said material to an oven, heating thematerial at a temperature of at least 500 F. and not more than 900 F. ata pressure slightly in excess of atmospheric pressure in an atmosphereof incompletely burned natural gas containing from 14% to 8% oxygenvarying inversely as the temperature and for a time ranging from about 4minutes to 1.25 minutes, and then immediately quenching in a continuousmethod for curing the rubber-like material.

6. In a method for continuously curing compounded vulcanizablerubber-like material including a vulcanizing agent, the stepscomprising; continuously shaping said uncured vulcanizable material,supplying a continuous length of said material to an oven, moving thematerial through the oven at a substantially constant rate so that thematerial remains in the oven for from 1 /2 minutes to 4 minutesdepending on the temperature, heating the material to a temperatureranging from 900 F. at a pressure slightly in excess of atmosphericpressure to 500 F. in an atmosphere which is non-oxidizing to thesurface of the rubber-like material and which contains oxygen inquantities of not in excess of 8% to 14% depending on the temperature,and then continuously cooling the part after curing in a non-oxidizingmedium.

'7. In a method for continuously curing com- 'ifipumledi vuieanizeble;rubberslike material. in:- eluding-avuleanizing agent, the steps comprismg; 'continuouslyshaping vulc'anizable material, supplying: a continuouslength of said materi'a-r'toan'oven, moving the material through theoven at a: substantially constant rate so that the-material remains intheoven-for from 1 minutes to 4 minutes depending on the temperatureheating the material to atemperature rangingfrom:900 F; to 500 F. at a.pressure slightly in excess" of: atmospheric pressure in 'an'atmospherewhich isl-noneo'xidizing to the surface of the rubber-like material, andY which contain-sox'ygen in quantities of .not. inexcess or; 81% to- 14%depending on the. temperature, andtthen continuously quenching the partin water.

8.- In the; method ofv curingeompounded rubber-vlikeamaterialsincluding. a vulcanizing agent under accelerated conditions; the-stepscomprising; shaping vulcenizable uncured rubber-like material,supplying; said, uncured compound to an; oven, supplying an--atmosphereto said oven consisting of a non-oxidizing gas at pressuresapproximating butslightly greater than atmospherie pressure and whereinthe oxygen eontent a, temperature between, :500? and 900 F; atapressureslightly in excess of atmosphericime51- sure for a time sufiicient tocure the material and .then immediately cooling the vulcanizedmae terialunder non-oxidizing conditions before ex: po'sure to the atmosphere.

. STANLEY R. FRANCE. I References Cited in the file of this patentUNITED STATES PATENTS Number Name Date 7 Re. 22,188 Minor-- Sept; 29,1942 1 ,289,043- Comstock- Dec; "24,1918 1,963,943 Gross June 19,- 19342,008,861 Gray Ju'1y 23; 1935 2290;550 Ge'ss1er' July 21, 1942 2549396Somerville Apr; 17, 1951' OTHER REFERENCES Vanderbilt, Rubber Handbook;1948, (.230-Park Ave" NewYork City) pg. 209,81;

1. IN THE METHOD OF CURING COMPOUNDED RUBBER-LIKE MATERIALS INCLUDING AVULCANIZING AGENT, THE STEPS COMPRISING; SHAPING VULCANIZABLE UNCUREDRUBBER-LIKE MATERIAL, SUPPLYING SAID MATERIAL TO AN OVEN, HEATING THEMATERIAL AT A TEMPERATURE OF AT LEAST 500*F. AND NOT MORE THAN 900*F. ATA PRESSURE SLIGHTLY IN EXCESS OF ATMOSPHERIC PRESSURE IN AN ATMOSPHEREOF INCOMPLETELY BURNED NATURAL GAS CONTAINING FROM 14% TO 8% OXYGENVARYING INVERSELY AS THE TEMPERATURE AND FOR A TIME RANGING FROM ABOUT 4MINUTES TO 1.25 MINUTES, AND THEN IMMEDIATELY COOLING THE VULCANIZEDPART UNDER NON-OXIDIZING CONDITIONS BEFORE EXPOSURE TO THE ATMOSPHERE.